Almost all electrical machines—motors, generators or transformers—use electrical steel to make a stack of laminations. Electrical steel is a thin steel strip with high magnetic permeability. Nominal thickness of coating can range from 0.8 to 8 μm (30 to 300 μinch) and is so thin (akin to surface roughness) that it is generally insulative below a certain voltage and resistive above that voltage, and hence sometimes it is also called a semiconductive coating. Electrical machinery makers punch and shape the steel strip into thin sheets called a lamination, recoat them if needed to improve the insulation quality, and stack them to make the core of an electrical machine. It is well known that a major purpose of insulative coating is to prevent flow of eddy currents from one lamination to next, as excessive eddy currents could cause overheating. The quality of insulation on the coated surface is characterized by surface insulation resistivity (abbreviated as surface resistivity herein). It is the resistance of a unit area of coating, i.e., product of volume resistivity and thickness of the coating. The rationale is that if the surface resistivity is sufficiently high, then the eddy currents that are generated will be relatively small, thereby preventing overheating. With this in mind, a standard test apparatus, termed Franklin tester (see ASTM standard A717-06), was developed to measure the surface resistivity of a single lamination or steel sheet. FIG. 1 and FIG. 2-A show the prior art of testing the insulation characteristics of coated surfaces of electrical steel sheet.
FIG. 1 shows a schematic of the Franklin tester, which comprises a test head and a test head power supply. The test head applies a specified pressure P on the test specimen 1 using two parallel rows of five vertically mounted steel rods 78 free to move axially against surrounding springs 77. Test specimen 1 comprises base metal 5 covered by a thin insulative coating 4 and can be an electrical steel sheet or a lamination. Metallic button electrodes 91 are mounted on each rod 78, and are insulated from the rod. Button electrodes 91 contact the insulated surface 4 at ten discreate areas; they press against the insulated surface 4 defining an imprint 90 of button electrodes on the insulated surface 4 as shown in FIG. 2-A. A power supply 31 applies a potential of 0.5 volt between the electrodes 91 and the test specimen's base metal using a 5 ohm resistor 81 connected in series to each button. A fully conductive sheet yields a “Franklin current” of 1 ampere while a perfectly insulated sheet yields 0 amp. Thus Franklin current ranges between 0 and 1 Amp and indicates the quality of insulation of the coated surface. A simple formula is used to convert the Franklin current into surface resistivity.
Franklin tester is the defacto standard that is used by steel mills and electrical machinery manufacturers to measure the insulation quality of the coating on electrical steel or laminations. But several instances have been reported where Franklin current, measured by a steel mill's Franklin tester, is acceptable, but when measured by an electric machinery maker's Franklin Tester, is unacceptable. Poor repeatability and reproducibility of its results have cast doubts on its usefulness in measuring the insulation quality of electrical steels per Godec (1982).
The prior art has attributed this non repeatability to unequal contact areas of the ten discrete button electrodes; this is believed to be caused by faulty alignment of the ten electrodes, unequal contact pressures exerted on ten different electrodes resulting in uneven wear etc. One of the earliest attempt to achieve repeatability is for an operator to place a fine sandpaper between the ten electrode buttons and a flat plate while under pressure and then remove the sandpaper, thereby polishing the surface of all electrodes. Another method proposed in 1979 in U.S. Pat. No. 4,156,841 is to provide a pivoted tip to the contact electrodes so as to maximize the contact area. Another method proposed in 1982 in U.S. Pat. No. 4,360,774 replaces spring-biased buttons with non-depressable buttons. The intent of all these improvements was to equalize the contact areas of ten discrete electrodes thereby increase the repeatability.
One great benefit of Franklin tester is that it does account for degradation in surface resistivity due to the clamping pressure and temperature during operation. Surface resistivity of an isolated lamination free of clamping pressure can range from 1 to 200 ohm cm2 depending on the coating thickness. Clamping pressures can reduce it by nearly 50%. Operating temperatures further degrade it. Under combined stacking pressures and temperatures, coating can soften and become thinner, further reducing the surface resistivity. These effects are satisfactorily accounted for by the Franklin tester.
But even after equalizing the contact area and accounting for pressure and temperature effects, Franklin Tester still suffers from several limitations. One fundamental limitation is that it does not test the entire surface and hence is unable to detect shorts that may be present in untested areas of insulation which could lead to catostraphic core failure. Other concerns include, but not limited to, not accounting for variations in surface resistivity from one location to another, limited ability to apply sufficiently large voltage typical in large machines, and lacking provisions to follow crowned profile of laminations. These limitations resulted in Franklin tester's inability to distinguish between good quality insulation that can prevent core failure and bad quality insulation that can cause core failures, thereby rising questions about its usefulness. This created a demand for an improved test apparatus and test method that more accurately depects the quality of insulation which can prevent core failures.
That Franklin tester does not account for variations in surface resistivity can be seen from the following rationale. Surface resistivity is the product of the coating's electrical resistivity and thickness. While resistivity is a material property and hence is a constant, the thickness of the coating is not constant and varies from point to point. FIG. 2-B shows a zoomed view at location M of the coating 4. This mangified view shows that the coating thickness is non-uniform, and varies widely from point to point, from a maximum thickness tmax to a minimum thickness tmin. It is not unusual for coating thickness to vary by 50% to 90% or even 100%. In fact Marion-Pera (1994) found few spots with no coating, that is, iron shows on a coated surface in microscopic spots that are not seen by naked eye. Thus surface resistivity can vary by 50% or more depending on the location, and Franklin tester does not do a good job of accounting for this large variation, and it does not detect microscopic spots where there is no insulation and hence can cause electrical shorts.
That Franklin tester does not apply sufficient voltage stress on the surface insulation can be seen from the following rationale. The standard dictates that it should apply a relatively low voltage of 0.5 volts across coating. While this low voltage suffices for testing small or medium sized machines, it may be too low to test the surface insulation in lamination used in large machines. Large generators operate at very high voltage and power levels (voltage>20 kV, power>500 MW). Laminations in such high voltage machines are subjected to relatively high interlaminar voltages at core ends. At core-ends, interlaminar voltage may be 40 or 50 times higher than those in main body of the core (Anderson et al 1980) in steady state. During transient conditions, they may be two or three times higher. In fact, at the core-end, one generator manufacturer measured steady interlaminar voltage of ˜5 v peak-to-peak (Platt 1982). Another manufacturer estimated that, at the core end, steady interlaminar voltage will be more than 1V. In contrast, the Franklin tester applies only 0.5 V across laminations. Hence the voltage applied by Franklin tester is significantly lower and inadequate to test laminations of large generators. As a result, the core ends of such large machines are extremely vulnerable to failure by electrical shorts or bad quality insulation that is not detected by the Franklin tester.
That Franklin tester does not follow crowned profile of laminations can be seen from following rationale. It is well known that laminations are not uniformly thick; this nonuniformity is characterized by crown and edge drop, as described in U.S. Pat. No. 7,004,002. Crown refers to a gradual reduction in thickness as one moves from the width center towards edges of steel strip. For example, a 0.35 mm (0.014 inch) thick steel strip typically has crown of about 5 μm (200 pinch). This means that surface of lamination is not perfectly flat at any point, and is microscopically slanted. As a result, even if all contact buttons are perfectly flat, since the lamination surface is slanted, buttons do not completely touch the insulated surface. This mismatch in flatness leads to different buttons contacting with different areas, thereby resulting in scatter in test results.
That Franklin tester does not test a large area of surface insulation can be seen from FIG. 2-A. This shows a representative lamination 1 used in utility generators; it has a large area of back iron and a smaller area containing teeth. The total insulated area can be as high as 1300 cm2 (200 in2). Also shown is tested area 7 comprising ten discrete imprints 90 under ten discrete electrodes 91. Per ASTM standard A717-06, the total tested area 7 is 6.54 cm2 (1 in2). The balance area 9 is 1293 cm2 (200−1=199 in2) or 99.5% of insulated surface remains untested. The prior art attempted to overcome this untested area problem by repeating the Franklin test over 5 to 10 different areas and take an average. But ten tests only increase the tested area to about 10 in2, which still leaves 190 in2 or 95% of the insulated area untested.
Perhaps the most fundamental and serious limitation of Franklin tester is its inability to detect defective spots within the insulative coating that can create electrical shorts. Defective areas are localized microscopic spots where resistance of surface insulation is zero or near zero. These defective spots are so small and microscopic that they cannot be seen by naked eye. As already explained, more than 95% of the coated area is not tested. This large untested area 9 could contain single or plurality of defective spots 92, 93, 94. They may occur either on the surface, on the edge of a lamination or on a tooth corner where the act of punching leaves an elevated surface and coating could be chipped away in a microscopic spot. These surface insulation defects are very difficult to detect by naked eye, but they could cause havoc on the performance of the machine. Such surface insulation defects could be caused by wide and varied sources, such as conductive debris, lumps on surface, bared spots, thin coating, pin holes, embedded conductive particles, burrs, rised edges, bared insulation at tooth corners etc.
If the insulated surface has a defect, then eddy current freely travels from one lamination to next. Shorting of two laminations effectively doubles lamination thickness so quadruples the eddy current. Shorting of three laminations increases eddy currents by a factor of 9. Such sharp amplification of eddy currents in confined and localized areas leads to concentrated heat; as this concentrated heat cannot be dissipated in confined areas, it leads to hot spots. Local temperature of such hot spots is builds up to such high value that at some point it burns the coating, melts the iron and welds the laminations. Surface insulation defects hence can lead to devastating core failure.
Published literature on generator core failures summarised below indicates that one major cause is interlaminar shorts caused by defects in the insulation coating. In 1968, in a power plant operated by CEGB (currently National Grid) Britain, a 500 MW generator core was damaged by melting per Fairney (1989). In a simulation of this 500 MW generator, Tavner and Anderson (2005) predicted that currents as high as 11,500 Arms could circulate due to an interlaminar short. In 1998, a 300 MW generator in San Antanio, Tex., overheated because of numerous interlaminar shorts and was removed from service leading to economic losses per Spisak (2004). In 2000, core failure occurred in a 415 MW generator in Castle Dale, Utah, in which about 200 pounds of molten iron flowed out from the end of the stator core, the cause for which was traced to a small interlaminar short which grew into a major melt zone per Edmonds et al (2007).
Thus even though the surface insulation in areas exposed to Franklin tester's contact buttons has high surface resistivity—as seen by low or near zero Franklin currents—defects in the surface insulation could lead to core failure. From this discussion, it is apparent that even if a lamination passes Franklin tester, it does not necessarily mean that a defective surface insulation protects the machine from core failure. Because of inadequacy of Franklin tester to detect defective insulation, there is a strong need for an surface insulation tester that could detect the failure-prone defects within the insulation coating and reject bad quality laminations. This invention described herein aims to fill this need, thereby reducing chance of core failure and resulting outages and economic loss. The present invention is not restricted to measuring surface resistivity. It can also measure other characteristics of an insulative coating, such as number of defects per unit area, dielectric strength etc.